1. Field of Invention
The present invention relates to an aluminum nitride substrate for use in a light-emitting diode and, more particularly, to a method for making a pure aluminum nitride substrate.
2. Related Prior Art
Currently, high-power LED devices convert 20% of electricity into light at most while converting the remaining 80% of the electricity into heat. The lives and efficiencies of the LED devices and the stability of the materials for packaging the LED devices would considerably be affected should the heat not be removed from the LED devices. Therefore, heat management is an important issue for the LED devices.
An LED device includes a substrate for supporting at least one LED chip. The material and quality of the substrate influence the performance of the LED device such as the luminance, efficiency and life. For a low-power LED, a Thermal conductivity aluminum oxide substrate is good enough. However, as the LED industry develops and the market grows, there is a growing demand for the high-power LED devices. However, the thermal conductivity aluminum oxide substrates heat cannot transfer from the high-power LED devices fast enough. Therefore, the thermal conductivity aluminum oxide substrates cannot be used in the high-power LED devices. To solve the problem related to the heat radiation from the high-power LED devices, attention has been paid to highly thermally conductive substrates that are made of materials with high K values, i.e., high heat transfer coefficients, so that heat can effectively be transferred from the LED chips. Heat generated by the LED chips when the LED chips operate to emit light is transferred to the substrates on which the LED chips are provided, and the heat is subsequently released to the environment from the substrates.
The conventional high-power LED devices include GaN substrates and Al2O3 substrates that emit blue light. The Al2O3 substrates are however poor regarding the heat transfer, and their heat transfer coefficients are 17 to 27 W/mK. Therefore, driven by large currents, the conventional LED devices encounter a serious problem related to heat radiation.
To solve the foregoing problem, efforts have been made to develop highly thermally conductive aluminum nitride (“AlN”) substrates. The aluminum nitride substrates are highly thermally conductive, and their heat transfer coefficients are 170˜250 W/mK. Furthermore, the aluminum nitride substrates are electrically isolative, erosion-resistant and refractory. The lives of the aluminum nitride substrates are long, and the physical properties of the aluminum nitride substrates are stable. Therefore, the aluminum nitride substrates can be used in the high-power electronic devices.
Conventionally, the aluminum nitride substrates are limited to 4.5-inch aluminum nitride substrates for at least two reasons. Firstly, it is difficult and hence expensive to produce the aluminum nitride substrates. Secondly, it is difficult to control the quality of the aluminum nitride substrates. For example, the aluminum nitride substrates are vulnerable to cracks due to uneven heating during the sintering. Hence, it has not been any successful attempt to produce 8-inch wafer-level aluminum nitride substrates.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.